title: Heteroscorpionate rare-earth initiators for the controlled ring-opening polymerization of cyclic esters.
abstract: A series of neutral rare-earth metal amides containing different achiral and chiral heteroscorpionate ligands was synthesized and characterized and these compounds were employed in the polymerization of cyclic esters. Thus, treatment of [Ln{N(SiHMe(2))(2)}(3)(thf)(2)] (Ln = Nd, Sm) with acetamide or thioacetamide heteroscorpionate ligands for 2 h at 0 °C afforded the α-agostic silylamido dimeric rare-earth compounds [Ln{N(SiHMe(2))(2)}(NNE)](2) (Ln = Nd and Sm; NNE = heteroscorpionate ligands, E = O, S) (1-8), some as enantiopure complexes. Complexes 1-8 contain dianionic heteroscorpionate pseudoallyl ligands resulting from C-H activation of the bridging methine group of the bis(pyrazol-1-yl)methane moiety and subsequent coordination to the metal center. However, when the reaction was carried out for 1 h at lower temperature new bis(silylamido) dimeric lanthanide compounds [Ln{N(SiHMe(2))(2)}(2)(NNE)](2) (Ln = Nd and Sm; E = O) (9 and 10) were obtained. The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structures of 1, and 4 were also established. Neodymium complexes are active initiators for the ring-opening polymerization (ROP) of lactide (LA) and lactones, giving rise to medium-high molar mass polymers under mild conditions and with narrow polydispersities. These complexes were well suited for achieving well-controlled polymerization through an insertion-coordination mechanism. Achiral and racemic complexes did not affect stereocontrol in the polymerizarion of rac-LA but the enantiomerically pure complex 1 was found to exhibit a homosteric preference for one of the two enantiomers of rac-LA at low conversions.
authors.affiliation: Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, 13071, Ciudad Real, Spain. Antonio.Otero@uclm.es
authors.forename: Antonio
authors.initials: A
authors.lastname: Otero
authors.forename: Agustín
authors.initials: A
authors.lastname: Lara-Sánchez
authors.forename: Juan
authors.initials: J
authors.lastname: Fernández-Baeza
authors.forename: Carlos
authors.initials: C
authors.lastname: Alonso-Moreno
authors.forename: Isabel
authors.initials: I
authors.lastname: Márquez-Segovia
authors.forename: Luis F
authors.initials: LF
authors.lastname: Sánchez-Barba
authors.forename: José A
authors.initials: JA
authors.lastname: Castro-Osma
authors.forename: Ana M
authors.initials: AM
authors.lastname: Rodríguez
mesh_terms: D000577:Amides / Q000737:chemistry; D002384:Catalysis; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry; D018360:Crystallography, X-Ray; D004952:Esters; D006571:Heterocyclic Compounds / Q000737:chemistry*; D007700:Kinetics; D028581:Lanthanoid Series Elements / Q000737:chemistry; D008024:Ligands; D008674:Metals, Rare Earth / Q000737:chemistry*; D008968:Molecular Conformation; D009354:Neodymium / Q000737:chemistry; D058105:Polymerization; D013237:Stereoisomerism
chemical_list: D000577:Amides; D056831:Coordination Complexes; D004952:Esters; D006571:Heterocyclic Compounds; D028581:Lanthanoid Series Elements; D008024:Ligands; D008674:Metals, Rare Earth; D009354:Neodymium